Not applicable.
Not applicable.
The present invention relates to medical devices, and in particular, to high pressure resistance mechanisms for devices which employ cryogenic fluids.
Recently, the use of fluids with low operating temperatures, or cryogens, has begun to be explored in the medical and surgical field. Of particular interest are the potential use of catheter based devices, which employ the flow of cryogenic working fluids therein, to selectively freeze, or xe2x80x9ccold-treatxe2x80x9d, targeted tissues within the body. Catheter based devices are desirable for various medical and surgical applications in that they are relatively non-invasive and allow for precise treatment of localized discrete tissues that are otherwise inaccessible.
A cryogenic device uses the energy transfer derived from thermodynamic changes occurring in the flow of a cryogen therethrough to create a net transfer of heat flow from the target tissue to the device, typically achieved by cooling a portion of the device to very low temperature through conductive and convective heat transfer between the cryogen and target tissue. The quality and magnitude of heat transfer is regulated by the device configuration and control of the cryogen flow regime within the device.
Structurally, cooling can be achieved through injection of high pressure cryogen through an orifice and subsequent expansion of the cryogen in an expansion chamber in the near-field of the orifice. For example, cryogen supplied at high pressure, ranging up to 800 psia, is generally a liquid-vapor mixture as it travels through a device to the orifice. Upon injection from the orifice, the cryogen undergoes two primary thermodynamic changes: (i) expanding to low pressure and temperature through positive Joule-Thomson throttling, and (ii) undergoing a phase change from liquid to vapor, thereby absorbing heat of vaporization. The resultant flow of low temperature cryogen through the expansion chamber acts to absorb heat from the target tissue proximate to the expansion chamber, and thereby cools the tissue to the desired temperature.
Of the two processes contributing to the cooling power of the device, evaporative boiling through a change in phase creates a far greater cooling effect through the absorption of latent heat of vaporization, on a specific basis, than that of Joule-Thomson cooling alone. Therefore, it is highly desirable to supply the device with a cryogen that is as much in liquid rather than gaseous phase, before the fluid is injected into the expansion chamber to cool tissue. However, during transit through the device, such as through an elongate catheter, the cryogen supplied typically passes through a region of comparatively high temperature, such as a region of the human body preceding the target area, and is thereby warmed. This warming, coupled with head losses in the flow of cryogen down a length of several hundred diameters of tubing, acts to degrade the quality of cryogen from its high pressure liquid form to a lower pressure, higher temperature, mixed phase form, leading to significantly degraded cooling power of the device.
Therefore, it is desirable to insulate the flow of cryogen as it is supplied from the proximal to the distal end of the device, so as to prevent the source cryogen from warming before it undergoes thermodynamic cooling.
Another problem presented in such a cooling process is that the cryogen vapor which rapidly cools in the expansion chamber may, if the resultant pressure drop is extreme enough, sublimate or precipitate if the pressure drops below that of the triple point for the cryogen. This sublimation naturally degrades the cooling power of the device, as heat transfer is drawn from the cryogen vapor into the cryogen particulate, rather than from the tissue proximate the device into the vapor. Worse, sublimation leads to unsteady flow, non-uniform density, and unstable temperature and non-equilibrium conditions. The sublimed particles may also block the flow of cryogen in the relatively small lumens, thereby creating dangerous high pressure conditions in the tip.
The cooling power of the device is directly related to the temperature drop in the expansion chamber, which is in turn a function of the pressure drop in the expansion chamber. While it is therefore desirable to reduce the pressure of the expanding cryogen as much as possible so as to benefit from the corresponding gas-dynamic cooling thereby created, care must be taken to avoid dropping the pressure below the triple point. Thus, it is desirable to create conditions in the expansion chamber where a maximum amount of cryogen flow is expanded to the lowest possible temperature, but at a pressure above the triple point. This may be most practically achieved by regulating the xe2x80x9cback pressurexe2x80x9d of the device, i.e. by fine-tuning the pressure conditions downstream of the expansion chamber, so as to create a nominal pressure in the expansion chamber which is higher than the triple point of the cryogen flowing therethrough.
Furthermore, because the catheter based device is to be inserted into a body lumen or other internal region of the human body, the device must maintain a fluid seal, lest potentially damaging cryogen leak during application of the device. As enumerated above, the cooling power of the device is dependent on achieving the maximum flow of high pressure liquid phase cryogen through the device, so that the maximum possible cooling occurs in the expansion chamber. Because the cryogen is injected into the expansion chamber through a choked orifice, the resultant pressure of the cryogen flowing in the expansion chamber is positively correlated to the source pressure and flow rate of the supplied cryogen. Therefore, increasing the flow rate and pressure of the supplied cryogen correspondingly increases the pressure of the resultant cryogen flow in the expansion chamber.
To contain the cryogen in the expansion chamber, the structural properties of the device must be sufficient to properly seal the device and withstand the operating pressure of the cryogen flowing therein. Thus, the device must be optimally designed to provide for a maximum amount of cryogen flow while maintaining its structural integrity.
It is therefore desirable to provide a medical device which maximizes the cooling power of the flow of cryogenic fluid therethrough, namely through maintaining a steady, uniform supply of high pressure cryogen in liquid phase. It is also desirable to provide a medical device which minimizes cooling losses in the flow of cryogen as it is applied to tissue, as well as maximizing the ratio of the cooling power of the device versus its internal flow lumen diameter. Finally, it is desirable to provide a structurally sound expansion chamber with a maximum possible operating pressure, so that the maximum possible cooling may occur therein.
The invention discloses a cryogenic medical device with high pressure resistance tip, and a method for cooling the same.
In one embodiment of the invention, the medical device comprises a first member defining an injection lumen, a second member circumferentially disposed around the first member to define a return lumen therebetween. The return lumen has at least one cross-sectional area. A third member is disposed between the second member and the first member to define a restriction lumen between the third member and the first member. The restriction lumen has at least one cross-sectional area smaller than the at least one cross-sectional area of the return lumen. In another embodiment of the invention, the medical device comprises an elongate injection tube having a proximal end portion having at least one proximal orifice, and a distal end portion having at least one distal orifice, and an elongate catheter tube circumferentially disposed around the injection tube and defining a return lumen therebetween. The catheter tube has a distal end portion, the distal end portion being coupled to a thermally transmissive element, where the thermally transmissive element circumferentially encloses the distal end portion of the injection tube. A restriction tube is circumferentially disposed inside of the catheter tube and encloses a portion of the return lumen proximate the thermally transmissive element.
Finally, a method for cooling the cryogenic medical device is disclosed. The method includes the steps of: (i) providing a supply of cryogen at a pressure of at least two atmospheres absolute pressure in a storage container; (ii) fluidly connecting said supply of cryogen with a catheter having a first lumen inside of a second lumen, and a thermally transmissive element; (iii) providing a flow regulation system to dispense cryogen into the first lumen and to reduce the pressure in the second lumen to below one atmosphere absolute pressure; (iv) controllably injecting said supply of cryogen through the first lumen in proximity to the thermally transmissive element; and (v) providing a third lumen inside of the second lumen, the third lumen being proximal to the thermally transmissive element, the third lumen having a cross-sectional area smaller than the cross-sectional area of the second lumen.